This invention relates to the preparation of a highly densified boron carbide ceramic body by the pyrolysis of a mixture comprising boron carbide powder and a preceramic organosilicon polymer selected from the group consisting of polysiloxanes, polysilazanes, polysilanes, metallopolysiloxanes and metallopolysilanes. Such highly densified ceramic bodies can be prepared by sintering under pressure or by utilizing a pressureless process.
The products and processes of the present invention have several distinct advantages over prior art methods. (1) The green bodies have high strengths, thus, facilitating handling and machining before sintering. (2) The composition of the preceramic mixture can be varied to accommodate various molding techniques such as press and sinter or transfer/injection molding and sinter applications. (3) The green bodies of the invention are more dense and, thus, yield ceramics which have less shrinkage and greater sintered tolerance control upon pyrolysis.
Boron carbide ceramic bodies are well known in the art. They have found particular utility in wear parts, armor and the nuclear industry because of their high modulus, high hardness, low density, resistance to wear and nuclear properties. Early methods for producing these bodies involved hot-pressing boron carbide powder at temperatures up to 2300.degree. C. These methods, however, are relatively complex in that they require the use of high pressure during sintering and they often fail to produce bodies with high density. Additionally, these methods do not produce green bodies and are of limited utility in forming bodies of complex size and shape.
Numerous authors and inventors such as Dole et al. in "Densification and Microstructure Development in Boron Carbide", Ceram. Eng. Sci. Proc, pp 1151-1160 (1985), Schwetz et al. in U.S. Pat. No. 4,195,066, Henney et al. in UK Patent Application GB 2,014,193, and Schwetz et al. in J. Less Common Metals, 82, 37-47 (1981) describe achieving high density boron carbide ceramics by the pressureless sintering of a mixture comprising boron carbide powder and carbon. As disclosed therein, the authors/inventors mixed between 0.5 and 12% free carbon in the form of Novolac resin, phenolic resin, graphite, sugar, or polyvinyl alcohol with boron carbide powder and sintered to temperatures of 2100.degree.-2300.degree. C. to obtain ceramic bodies with densities .gtoreq.97%. These references, however, do not describe the inclusion of organosilicon polymers as the source of free carbon.
Weaver in U.S. Pat. No. 4,320,204 teaches the formation of high density (&gt;94%), refractory ceramics by the pressureless pyrolysis of 60-98% boron carbide powder 2-40% silicon carbide powder and, optionally, up to 10% aluminum. This reference, however, does not teach the use of organosilicon polymers as the source of SiC.
Prochazka et al. in U.S. Pat. No. 4,081,284 and Schwetz et al. in U.S. Pat. No. 4,524,138 teach the formation of high density ceramics by pressureless sintering varying amounts of boron carbide powder, silicon carbide powder and free carbon. Neither reference, however, describes an organosilicon polymer as the source of SiC and/or free carbon.
Bougoin et al. in "Pressureless Sintering of Boron Carbide with the Addition of Polycarbosilane", J. Mat. Sci., 22, pp 109-114 (1987) describe mixing polycarbosilane, boron carbide powder and a source of free carbon to form a mixture, pressing the mixture to form a green body, and sintering at 2175.degree. C. Sintered bodies with densities .gtoreq.92% were thereby obtained. This reference, however, does not describe the inclusion of organosilicon polymers other than the polycarbosilane. Moreover, the reference clearly states that the addition of an organic carbon source is necessary for ultimate densification.
Japanese Patent 53073000 describes a neutron absorption shield material consisting of a polyorganosiloxane, a crosslinking agent, a crosslinking catalyst and boron carbide powder. This reference, however, is not directed to the formation of high density ceramics. Thus, the reference doesn't describe sintering the above mixture to the elevated temperatures claimed herein.
The present application discloses for the first time that high density, high strength boron carbide ceramic products may unexpectedly be obtained by sintering a mixture comprising a preceramic organosilicon polymer and boron carbide powder.